This invention relates to an electrode plate which has a substrate and a multi-layered conductive film and which can be applied to a reflection preventing film, electromagnetic wave shielding film, transparent type or reflective type electrode for solar battery or electrode plate for a display device such as a liquid crystal display device or EL (electroluminescence) display device and a method for manufacturing the electrode plate.
A transparent electrode formed by arranging a transparent conductive film for permitting light of predetermined electrode pattern to pass therethrough on a glass substrate, plastic substrate or substrate on which semiconductor elements are formed is widely used for display electrodes of various types of display devices such as a liquid crystal display, an input/output electrode which permits an input to be directly effected on the display screen of the display device and the like.
As a liquid crystal display device using the transparent electrode, it is generally to use a transmission type liquid crystal display device containing a light source (lamp) as a back light. In the transmission type liquid crystal display device, since the power consumption by the back light lamp is large and thus the service life is short in the case of battery driving, the portability which the liquid crystal display device originally has cannot be fully utilized. For this reason, at present, a reflective type liquid crystal display device using ambient light (that is, containing no back light lamp) is actively developed.
FIG. 5 shows a reflective type liquid crystal display device 50 which is formed of a reflective type electrode plate 51 and a transparent electrode plate 52 with sandwiching LCD 509. The reflective electrode plate 51 is adhered to the transparent electrode plate 52 via a seal 510 such that the transparent electrode 507 faces the transparent electrode 505.
In the reflective electrode plate 51, a reflection film 502 and color filter 503 are sequentially formed on the surface of a back substrate 501 comprised of, for example, glass which faces a liquid crystal 509. A protection layer 504 for protecting and the leveling the surface of the color filter 503 and a transparent electrode 505 are sequentially formed on the color filter 503.
On one surface of the transparent plate, such as a glass plate 511, a polarizing film 508 is laminated. On the other surface of the glass plate 511, an array of transparent electrode 507 with TFT (thin film transistor)s 506 is formed. The color filter 503 is formed of plural pixels of light transmission type (which are hereinafter simply referred to as pixels) colored in R (red), G (green) and B (blue) and arranged in a predetermined pattern. The reflection film 502 is also used as a reflection electrode which can be used as a liquid crystal driving electrode in some cases.
In the conventional case, a thin aluminum film is widely used as the reflection film 502 formed on the back substrate 501. This is because aluminum is a metal having a large reflectance of light in the visible region, but recently, it is proposed to use silver as a material of the reflection film from the viewpoint of enhancement of the reflectance and a problem that a lowering in the reflectance of aluminum due to contact with the liquid crystal or glass substrate occurs.
However, the reflectance of silver itself is larger than aluminum by approx. 10%, but it has the following main defects when it is used to form a thin film of the electrode plate.
First, the adhearability thereof to the substrate of a material such as glass or plastic is low and it is easily separated from the substrate when it is formed on the substrate as a thin silver film. Particularly, when an electrode is formed on the substrate such as a glass plate, an SiO.sub.2 film is previously formed on the substrate and a silver-based layer is formed on the SiO.sub.2 film in some cases in order to prevent nebula of silver (or prevent the silver-based layer from becoming slightly opaque) due to migration of alkali metal from the substrate. At this time, since the adhesion between the SiO.sub.2 film and the silver-series thin film is poor, it is necessary to form an adhesion layer formed of a thin oxide film between the SiO.sub.2 film and the silver-series thin film. Therefore, the manufacturing process becomes complicated and the manufacturing cost is increased.
Secondly, a silver-series thin film formed of highly pure silver on the substrate tends to aggregate and become opaque by the influence of heat and oxygen and the reflectance of light tends to be lowered.
Thirdly, in a case where the thin silver film is exposed and made in direct contact with the outside air, silver sulfide or silver oxide is formed on the surface of the thin silver film and the thin silver film becomes discolored and the reflectance thereof is lowered.
Therefore, as the technique for solving the above problem and defects, the technique for forming a three-layered conductive film having a thin silver film disposed between oxide layers is proposed in U.S. Pat. No. 5,667,853 by the inventors of this invention.
In the above proposal, in a case where the transparent electrode for the transmission type liquid crystal display device is formed by use of the three-layered conductive film, the upper side oxide layer (oxide layer formed on the upper surface of the thin silver film) is formed in an amorphous state and the upper and lower oxide layers are formed with a slightly large film thickness of approx. 40 nm in order to attain the optimum optical characteristic. The reason why the upper side oxide layer is formed in the amorphous state is to prevent that silver atoms move along the grain boundary when crystals or grains are present in the oxide layer and the silver-based layer is aggregated or becomes opaque and the reflectance or transmissivity is lowered.
However, the above proposal has the following problem.
When the three-layered conductive film of the above proposal is patterned by the photolithography process by use of an etching solution, contact corrosion due to contact between different types of metals occurs, damage due to the etching process (particularly, damage to the interface between the thin silver film and the oxide film) is large, and particularly, the upper side oxide layer may be easily separated.
Further, in order to form a stable amorphous film as the oxide film, a mixed oxide layer having different types of oxide materials mixed together is used in some cases. But in this case, the electrical connection resistance of the conductive film becomes high and it is not desirable as the conductive film. Further, as described before, since damage to the interface between the thin silver film and the oxide film occurs, the reliability such as humidity resistance is greatly lowered and it does not reach the practical level.